Hand-held aerosol canisters are filled with a fluid to be dispensed through a nozzle by a propellant gas. Frequently, the fluid is dispensed as a finely divided spray, but may alternatively be dispensed as a foam or a relatively thin jet. A typical aerosol canister has a valve mounted at its upper end, to which a delivery mechanism is mounted. The valve often has a short projecting tube (a valve stem) on which the delivery mechanism is mounted, although sometimes the delivery mechanism may project into the canister through an aperture to make contact with the valve. A dip tube usually extends downwardly from the valve into the canister, through which fluid is forced by a propellant when the valve is opened.
Small button actuators with nozzles are known for use with such aerosol canisters. Some button actuators release fluid as a result of downward pressure on the actuator, while others release fluid in response to sideways pressure on the actuator (depending on the type of valve used). The button actuators can be installed by being pushed down onto the valve stem (due to a friction fit of the valve stem in a lower bore of the button actuator).
A separate overcap typically fits onto the aerosol canister to protect the button actuator from accidental operation. The use of a separate cap, however, which has to be removed before the button actuator can be accessed, is inconvenient. To overcome the disadvantages of a separate cap, overcaps have been developed that incorporate actuator features so that they can be left in place on the canister when the aerosol dispenser is in use. Such caps, however, can be accidentally actuated during non-use (e.g., during packaging or shipment) and thereby cause inadvertent discharge of pressurized fluid from the canister.